Each of the several techniques for nondestructive object testing, namely, leakage flux detection, eddy current measurements and ultrasonic wave examination, has unique applicability in comparison with the others in accordance with the diverse parameters of that particular testing situations. However, the application of ultrasonic techniques inherently is more difficult than the other methods based principally upon the need in present commercial ultrasonic apparatus for a fluid couplant medium between the object under test and a piezoelectric transducer transmitting and receiving the ultrasonic wave. Thus, in certain instances, in which one would prefer to employ the ultrasonic technique, he elects one of the other practices or foregoes testing if the use of a couplant is not permitted.
Observation of the phenomenon of electromagnetically inducing an acoustic wave in an object under steady-state magnetic field influence as described, for example, in U.S. Pat. Nos. 3,460,063 and 3,583,213, has not given rise to any substantial change on the commercial ultrasonic testing scene, despite elimination of the bothersome couplant medium. Thus, while a wide variety of EMAT (electromagnetic acoustic transducer) systems have become known, such as that shown in the '063 and '213 patents, SAW (surface acoustic wave) transducers described in the literature, and the like, the industry-wide impact thereof is not readily seen. From applicants' viewpoint, the industry presently lacks a commercially viable system environment for effective use of EMAT probes in the testing of moving objects, particularly thin-walled pipes and the like.
Two aspects of EMAT object testing, as constrasted with leakage flux testing, are considered particularly relevant to this situation. Peculiar to EMAT test apparatus is the need for it to provide a strong magnetic bias field at the transduction surface, i.e., the object surface to which the electromagnetic acoustic wave is applied. Conversely, leakage flux detecting transducers are passive, i.e., do not generate but simply detect flux issuing at an object flaw. Accordingly, air gap minimization and constancy is more critical to EMAT test apparatus. Further, leakage flux detecting transducers may include protective metal between detector coils and the object, which metal may ride upon the object during testing and otherwise protect the transducer from damage by the object during insertion of the object for testing. To the contrary, the EMAT probe with its transmitting and receiving coils must be contiguous with the transduction air gap, without intervening metallic protection. Given these aspects of EMAT object testing, some basis is perceived for the commercially lagging status thereof as against systems of the other testing categories above noted.